1. Field of the Invention
This invention relates to seismic data acquisition and to methods of processing seismic data. In another aspect, this invention relates to a process for the filtering of sideswipe noise or interference from seismic data. In still another aspect, this invention relates to a process for determining the number of interfering noises, and their respective directions or dips of arrival.
2. Description of the Prior Art
Seismic exploration involves generating seismic pulses at the surface of the earth by means of one or more seismic sources. The seismic pulses travel downwardly into the earth with a fractional amount being reflected and/or refracted due to differences in elastic properties at the interface of various subterranean formations. Detectors, such as seismometers, geophones or hydrophones, produce analog electrical seismic signals or seismic trace signals in response to detected seismic wave reflections and/or refractions. The analog electric seismic signals or seismic trace signals from the detectors can then be recorded. Alternatively, the analog seismic signals or seismic trace signals from the detectors can be sampled and digitized prior to being recorded. The seismic data recorded in either manner are subsequently processed and analyzed to determine the nature and structure of the subterranean formations.
From the recorded data, a seismic section is generated. A seismic section is a seismic image depicting the subsurface layering of a section of earth along a seismic line of profile. It is an important tool which the geologist studies to determine the nature of the earth's subsurface formations. However, before an array of seismic samples can be converted into a seismic section which can be interpreted by the geologist, the seismograms must be processed to reduce the degradation due to noise.
Recorded seismic data always includes some unwanted energy in addition to the desired reflections and/or refractions from the subterranean formations. This unwanted energy can distort or even ruin the seismic data by obscuring or masking seismic events related to the reflections and/or refractions from the subterranean formations. Unwanted energy can also create the appearance of geologic events that are in fact not real. It can be generated in many ways, such as atmospheric electromagnetic disturbance, wind, motor vehicle traffic, boat noise, system generated noise, and reflections from out of plane events.
In the ideal acquisition of two dimensional seismic data, the energy gathered by the receivers would all come from a two dimensional plane which is along the seismic line of profile encompassing the energy source and receivers wherein the two dimensional plane is generally normal to the earth's surface. Unfortunately, many times unwanted out-of-plane energy is gathered by the receivers.
This out-of-plane energy, commonly known as "sideswipe", is frequently encountered in marine seismic data acquisition, and is also known to occur in land based seismic data acquisition.